The present invention relates to a process for stripping and sterilizing the internal surface of a container, for example made of wood, metal, concrete or any other material, especially a wooden cask having a surface layer of a coating of organic and/or mineral material, especially a coating of tannin resulting from the maturation of a wine in a cask, as well as to a device for its implementation.
Wooden casks will be of particular interest in the remainder of the description, but it should be well understood that the invention is in no way limited thereto and that it can be applied to any type of container, whatever it is made of.
During the period of maturing wines in a barrel, it is generally accepted that the wood transfers various substances, such as furans, lactones, aldehydes, phenolic acids, phenols and ketones to the wine. The barrel puts the wine in an oxidizing balance and acts as a kind of micro-doser of oxygen, which allows a first oxidation-reduction aging of the wine. It is generally assumed that a new barrel transfers tannic substances to the wine while an old barrel transfers substances from the decomposition of the wood. A one-year-old barrel, i.e. a barrel having already served to mature a wine for one year, generally gives a taste of pure wood to the wine, while a six-year-old barrel generally gives a rancid taste.
To mature certain wines, wooden barrels, the inside of which has been scorched on the surface, are also used to transfer other substances, such as phenolic compounds, furanic aldehydes and color, to the wine.
Usually a barrel may serve to mature up to four wines over a period of 4 to 6 years. After this period, the barrel can no longer serve as a maturing tool since the wine has penetrated by about 5 to 10 mm into the thickness of the barrel which has a thickness of about 22 to 27 mm, this penetration of the wine causing sealing of the wood pores by the tannin coatings and by the alterations in the compounds of the wood, such as the phenolic compounds, tartaric acid, etc., which prevents the subsequent transfers of substances between the wood and the wine, which transfers are essential to the maturation of the wine. These old barrels may still serve as storage containers, but this is not usually the case, since microbe-related accidents may arise during storage, between the wine and the coatings covering the internal surface of the barrel.
For the maturation barrels, the quality of the wood used is very important and, in French vineyards, the wood used generally comes from oaks of about 150 to 300 years of age, which therefore have a very long renewal time faced with a very greatly increasing recent demand.
In order to reduce the cost of the barrels and to save the limited national heritage in oaks, a process for renovating the barrel has already been proposed.
One solution consists in carrying out a mechanical stripping, using a plane or a sander, inside the barrel, then in possibly carrying out scorching, in order to regain the organoleptic nature which is characteristic of a new barrel. However, this solution is lengthy and expensive to implement and does not allow the barrel to be sterilized against microbial infections. Furthermore, such a mechanical stripping leads to removing an appreciable thickness, several millimeters, of the barrel, which limits the number of possible renovations.
Another solution consists in chemically cleaning the barrel, but this solution is very cumbersome to implement and expensive.
Furthermore, the current renovation processes give quite disappointing results for the quality of the wines, since stripping which is too intense leads to a xe2x80x9cplankxe2x80x9d taste by completely renovating the raw wood, while stripping which is too light has no effect.
Furthermore, during the renovation of the barrel, it is difficult to reproduce the initial traditional scorching, since when the barrel is too scorched, it develops strange characteristics.
The object of the invention is to propose a process for stripping and sterilizing the internal surface of a container, which is both simple to implement and which allows a very high number of renovations.
The invention is based on the principle of renovation by laser which ensures accurate and selective stripping at a controlled temperature, by photoremoval of the biological stains, for example fungal, mold, polychlorophenol and chloroanisole compounds, and/or mineral stains which are deposited over the internal surface of the container. Since the biological stains have physical characteristics which are different to those of the material forming the container, the heat increase during the absorption of the light produced by the laser will be faster in the coating of organic and/or mineral material than in the container, which makes it possible to remove the biological and/or mineral stains without causing a transfer of energy to within the material forming the cask.
For this purpose, the subject of the invention is a process for stripping and sterilizing the internal surface of a container made of wood, metal, concrete or some other material, having a surface layer of a coating of organic and/or mineral material, especially a coating of tannin resulting from the maturation of a wine in a cask, characterized in that it consists in applying, over the surface to be treated, pulsed radiation produced by an intense optical source, each pulse having a duration which is short enough and an energy density per unit area to be treated which is high enough to cause the sublimation of the said surface layer, the surface of the container thus stripped being sterilized by the heat released by the radiation. Using the invention, the layer of organic and/or mineral material is sublimed, which generates a gaseous plasma in the form of smoke, which avoids the drawbacks connected with the use of an aqueous solution.
Advantageously, each pulse has a duration of between 10 and 200 ns and an energy density of between about 6.5 and 9 J/cm2. Preferably, each pulse has a duration of about 100 ns and an energy density of about 8 J/cm2.
A long pulse duration, for example of the order of ms or us, would lead to a transfer of energy into the material forming the container and a low rate of ejection of the sublimed marks, while the organic and/or mineral coatings have to be removed over a small thickness, quickly and without consuming too much energy. With a. pulse duration of about 100 ns a very high peak value is obtained for the beam, which causes a high ejection rate of the sublime organic and/or mineral material and low diffusion of the heat into the material forming the container.
According to another characteristic, the process consists in applying, over each unit area, from 2 to 20 pulses, preferably between 4 and 10 pulses, depending on the type of material of the container to be treated, the state of the surface to be treated and the thickness of the organic and/or mineral coating.
According to another characteristic, the radiation is determined so as to cause a quasi-adiabatic sublimation of the layer of organic and/or mineral material on the surface to be treated. In particular, provision can be made for 80% of the heat produced by the radiation to be absorbed by the surface layer during sublimation, the remaining 20% being dissipated within the thickness of the material forming the container.
Preferably, each pulse causes the sublimation of about 20 xcexcm thickness of material surface to be treated.
Advantageously, the process consists in evacuating the gaseous plasma produced during the sublimation, by sucking it up or blowing it out using an inert gas or air.
According to another characteristic, the intense optical source is a laser source, for example a CO2 laser source at atmospheric pressure and with transverse excitation.
According to another aspect of the invention, for a wooden container, the process consists, simultaneously with or subsequent to the step of stripping and sterilizing, in applying over the surface to be treated a second intense optical radiation, the said second radiation being applied continuously or quasi-continuously for a duration which is long enough and with an energy density per unit area to be treated which is high enough to cause scorching of the wood on the surface. Advantageously this second radiation is applied by a laser source with a defocused beam or by beam scanning.
Preferably, the second radiation has a power density of between 100 and 200 W/cm2 for a duration of application of about 0.5 to 0.2 seconds. In this case, the second radiation preferably has an energy density per unit area to be treated of about 20 J/cm2.
Although the energy density received by the wood, in the case of scorching or toasting, is greater than that for stripping, the total energy is transferred over a long time during scorching, which means that the heat diffuses into the wood and chars it on the surface, while, in the case of stripping, the energy is applied over a very short time, causing instant sublimation of the organic layer.
In another variant, the second radiation is applied by an infrared or ultraviolet lamp, for example, a lamp having a power of 70 W for an application time of several minutes, with a distance of a few centimeters between the radiation source and the surface to be treated.
The subject of the invention is also a device for implementing the aforementioned process, characterized in that it comprises an intense optical source capable of producing pulsed radiation in order to strip and sterilize the internal surface of the container, a waveguide connected to the optical output of the source, an optical focusing head connected to the output of the waveguide, in order to define the cross section of interaction with the surface to be treated and thus the energy density to deposit per unit area, a robot for the relative movement between the optical head and the internal surface of the container to be treated, and a central control unit in order to control and synthesize, on the one hand, the source parameters such as the number of pulses to be applied per unit area, the impulse frequency and the radiation power of the source, and on the other hand, the movements to be carried out by the robot in order to treat the entire internal surface of the container.
Advantageously, the robot is capable of making the said optical head pivot through an angle of about 120xc2x0 with respect to the axis of the container.
According to another characteristic, the robot is capable of driving the optical head in relative rotation about the axis of the container with respect to the container.
According to yet another characteristic, the robot is capable in driving the container in relative axial translation with respect to the optical head, which may in this case be connected to a telescopic or extensible waveguide.
Preferably, the optical head is located at a distance from the surface to be treated of about a few tens of centimeters.
According to another aspect of the invention, the device comprises a camera for displaying the surface treatment, the said camera being connected to a display screen and to the central control unit in order to control the surface treatment visually and in real time.
According to another characteristic, the optical head is arranged so as to penetrate inside the container, for example by the bunghole of a cask made of wood, or by a hole specially made in one of the heading pieces of the cask or else by one of the ends of the cask from which the heading piece has been removed.
Advantageously, the device comprises a pipe for sucking up or blowing out smoke generated by the stripping.
The purpose of removing the gaseous plasma is to avoid, on the one hand, any recontamination of the treated surface and surroundings, on the other hand, any interference with the optical beam and with any display camera.
According to another characteristic, the device a second intense optical source for producing the second radiation for scorching a container made of wood.